1. Field of the Invention
The present invention relates to an ultrasonic (vibration wave) motor which generates a travelling vibration wave on a stator to drive a movable member, and more particularly to a driver therefor.
2. Related Background Art
In the past, control of the rotating speed of a vibration wave motor has been effected by changing an amplitude of the periodic voltage applied to a stator of the motor. However, since the periodic voltage is usually a high voltage (50-100 volts), a step-up circuit such as a DC-DC converter is necessary when the vibration wave motor is mounted on small portable equipment, such as a camera having a battery as an energy source, and the output voltage of the step-up circuit must be varied for application to the stator of the motor. In order to vary the output voltage of the step-up circuit, a plurality of zener diodes used as reference voltages of the step-up circuit are provided and one of them is selected. This arrangement is complex in configuration. Further, because the voltage is high, high voltage zener diodes are required. As a result, such a device is not suitable for integration and hence cannot be made into compact portable equipment.
The speed may be adjusted by changing the frequency of the periodic voltage from a resonance frequency of the motor. In this method, however, the voltage applied to the electrode may become so high that the stator may be broken.